Carbonaceous matter typically contains volatile compounds that make the carbonaceous material less desirable or useful than it otherwise would be for some purposes. Volatile compounds are those compounds that are given off as a vapor or gas by heating the carbonaceous material to a particular temperature. For example, volatile compounds or matter are given off by heating coal up to 950° C. under carefully controlled conditions and driving off the volatile compounds; the mass of which is determined by measuring the weight loss of the coal after the heating process. It typically does not include the weight loss due to water content, which is removed at 105° C.
Volatility is usually critical to industries that commonly use carbonaceous materials in their processes, for example coal for furnaces for steel making, coke manufacturing, and also for power generation. It is commonly known that volatility determines the burn rate of a particular coal. High volatility coal ignites easily, but it is not as desirable as moderately to low volatility coals because it does not contain as much energy per unit volume due to its high volatility. Many coals are “coked” prior to their industrial use by removing the volatile compounds through heating it to high temperatures in the absence of oxygen. Coking coal is used in the steel manufacturing process, where carbon must be as volatile-free and ash-free as possible. Calcining is another thermal treatment process that removes the volatile fraction of the carbonaceous material, such as coal. Typically, coking processes occur at temperatures of approximately 500° C. and calcining processes occur at temperatures of approximately 1,300° C. Both of these processes typically occur in inert atmospheres in the absence of oxygen. The problem with these types of processes is that they require high energy requirements and do not typically recover the volatile compounds as they are vented to the atmosphere or a gas scrubber of some type.
It has become increasingly important to recover the volatile compounds that are entrained in the carbonaceous material, such as coal For example, coal tar is a liquid having a high viscosity that has many uses including: fuel, pharmaceutical bases, shampoos, soaps, carbon fibers, waterproofing materials, and as a raw material for dyes, drugs, and paints. Coal tar may be extracted from coal by a variety of processes. For example, it is known to use solvent extraction processes to remove the volatile coal tar from coal.
In addition to calcining and coking coal, there exist several solvent extraction methods for removing the volatile compounds from carbonaceous material, such as coal. Typically, organic solvents, such as chlorinated organic solvents, are used to extract the volatile compounds from coal and then recovered during a distillation process. Many of these type solvents are hazardous materials and require substantial expenses related to implementing safety devices and measures for handling, storing, and using such solvents. In addition, efficient distillation of the volatile compounds from the organic solvents may not always be achievable.
U.S. Pat. Nos. 4,871,443 and 4,806,228 discloses a method for removing salts from coal tar and coal pitches. The method includes washing the coal tar/pitch in a pressure container with water and carbon dioxide gas at a temperature and pressure near the critical point of the gas. Recovery and removal of the solvent and the tar or pitch phase occurs upon reduction of the pressure. The methods are performed on an already extracted coal tar/pitch specimen and not on untreated carbonaceous material, such as coal.
U.S. Pub. Pat. App. No. 20070095753 discloses methods for removing residues from substrates using environmentally friendly solvents. Petroleum residue, including coal tar is removed from a petroleum-based substrate by dissolution in carbon dioxide miscible solvent, which dissolves a portion of the petroleum residue. The solvent is then separated and contacted with carbon dioxide such that the petroleum residue is precipitated. This reference discloses the use of environmentally friendly solvents, known as GRAS solvents.
U.S. Pat. No. 4,446,921 discloses a method for the underground gasification of solid fuels in which volatile compounds existing in the solid fuel can be recovered. Underground solid fuel is opened up with a super critical gas phase that dissolves water and the volatile organic compounds that would otherwise impede the later gasification process. The volatile organic compounds and water are later separated from the super critical gas phase above ground. It discloses that the gas enters the underground solid fuel at a temperature 10° C. to 100° C. above its supercritical temperature and at a pressure 2 bar to 300 bar above its critical pressure. This is performed on untreated solid fuel deposits, such as underground coal deposits.